In an optical communication in which information is transferred by a light signal through an optical fiber, the dispersion of the wavelength of the light signal occurs during the transmission. The dispersion of the wavelength causes waveform degradation of the light signal, so that faulty information may be transferred. As a result, a transmission distance in which correct information is transferred is limited by the wavelength dispersion. Therefore, it is necessary to narrow the wavelength spectrum of the light signal to lengthen the transmission distance. In order to obtain a light signal having a narrow wavelength spectrum, the external modulation method, in which a light signal emitted from a semiconductor laser is modulated by an optical modulator, is applied.
A conventional optical modulator has been described on page 4-193 of "Part 4, Spring term National Conference Record, 1989. The Institute of Electronics, Information and Communication Engineers of Japan", titled as "High performance optical intensity modulator monolithically integrated with a DFB laser".
The conventional optical modulator is an electroabsorption type optical modulator which is monolithically integrated with a semiconductor laser.
The optical modulator and the semiconductor laser are fabricated on a common substrate having a common electrode on a first surface thereof. The optical modulator comprises a light absorption layer among epitaxial layers grown on a second surface of the substrate, and a modulator electrode provided on a cap layer which is a top layer in the epitaxial layers, and the semiconductor laser comprises an active layer among epitaxial layers grown on the second surface of the substrate, and a laser electrode provided on a cap layer in the epitaxial layers, such that the absorptionlayer and the active layer are coupled by a butt-joint, and opposite facets of the optical modulator and the semiconductor laser are coated with anti-reflection layers.
In operation, the semiconductor laser emits a laser light having a uniform optical intensity and a uniform oscillation wavelength. Then, the optical modulator modulates the intensity of the light signal supplied by the semiconductor layer by changing the absorption coefficient of the optical modulator. By using the optical modulator, the dispersion of the wavelength spectrum of the light signal becomes one fifth of that in a former method in which a semiconductor laser emits a light signal modulated in intensity. Therefore, the transmission distance which is limited by the dispersion of the wavelength spectrum becomes five times as long as that in the former method.
In the conventional optical modulator, however, there is a disadvantage in that extension of the wavelength spectrum of the light signal still occurs due to a minute phase modulation generated during the intensity modulation by the optical modulator.